1. Field of the Invention
The present invention generally relates to a hydrodynamic bearing device utilizing a dynamic pressure and a method for manufacturing the device.
2. Background Information
In recent years, a storage capacity of a recording apparatus or the like using a disk or the like has been increasing, and a data transmission rate thereof has been increasing as well. A high speed and a high precision of rotation is necessary for a spindle motor that is used for such a recording apparatus. Therefore, a hydrodynamic bearing device is used for a rotation shaft portion of the spindle motor. A conventional hydrodynamic bearing device will be described below with reference to FIGS. 9 and 10.
FIG. 9 is a cross sectional view of a spindle motor including a conventional hydrodynamic bearing device. As shown in FIG. 9, a sleeve 101 having a bearing bore 101a is made of a sintered metal that is made of sintering powdered metal, such as a copper alloy. The sleeve 101 is made of a sintered metal mainly to reduce manufacturing costs. If the sleeve 101 is produced by machining a metal bar or the like, a lot of chips will be generated as waste material. In contrast, a sintered metal does not generate such chips. In addition, the time necessary for producing a sleeve using a sintered metal is a fraction of the time necessary for producing the same by machining. Accordingly, the production using a sintered metal is suitable for low-cost mass production.
On the outer surface of the sleeve 101, a sleeve cover 114 is provided. The sleeve cover 114 is made of a metal that is not a sintered metal. A shaft 102 is inserted in the bearing bore 101a of the sleeve 101 in a rotatable manner. A thrust flange 103 is fixed to a lower end portion of the shaft 102. The thrust flange 103 is housed in a space enclosed by the sleeve 101, the sleeve cover 114 and a thrust plate 104. A lower face of the thrust flange 103 in FIG. 9 is opposed to the thrust plate 104. An upper face of the thrust flange 103 is opposed to a lower end face of the sleeve 101.
A rotor hub 105 is fixed to an upper end portion of the shaft 102. A rotor magnet 106 is fixed to an inner surface of the rotor hub 105. The rotor magnet 106 is opposed to a motor stator 107 that is fixed to a base 108. An inner surface of the bearing bore 101a of the sleeve 101 is provided with dynamic pressure generating grooves 109a and 109b in the radial direction that are well known in the art. In addition, a face of the thrust plate 104 that is opposed to the thrust flange 103 is provided with dynamic pressure generating grooves 110a in the thrust direction that are also well known. Dynamic pressure generating grooves 110b may be formed on at least one of the opposed faces of the thrust flange 103 and the sleeve 101, if necessary. Oil 111, as working fluid, is filled in the space between the shaft 102 and the sleeve 101, including the dynamic pressure generating grooves 109a, 109b, 110a and 110b, as well as in the space between the thrust flange 103 and the sleeve 101 and the space between the thrust flange 103 and the thrust plate 104.
An operation of the conventional hydrodynamic bearing device will be described with reference to FIG. 9. When the motor stator 107 is supplied with power, a torque is generated by the rotor magnet 106, so that the rotor hub 105, the shaft 102 and the thrust flange 103 rotate as one body. As a result of this rotation, the dynamic pressure generating grooves 109a, 109b, 110a and 110b respectively give a pumping pressure to the oil 111 in the corresponding portions. Accordingly, radial bearings are formed at the area of the dynamic pressure generating grooves 109a and 109b for supporting the shaft 102 in the radial direction, while thrust bearings are formed at the area of the dynamic pressure generating grooves 110a and 110b for supporting the flange 103 in the thrust direction. Thus, the shaft 102 and the flange 103 rotate without contacting the bearing bore 101a and the thrust plate 104.
Since the sleeve 101 is made of a sintered metal, it has pores at 2-15% of volume (small spaces contained in the sintered metal). The pores include those called “tissue pores” existing inside the sintered metal and those called “surface pores” opening on the surface of the sintered metal. In a usual sintered metal, the surface pores and the tissue pores are communicated with each other. Although the sleeve 101 made of the sintered metal is impregnated with oil at a pressure lower than an atmospheric pressure in advance, the oil can pass through the sleeve 101 by moving in the pores. In the conventional example, the sleeve 101 is surrounded by the sleeve cover 114 so that the oil does not leak externally by moving through the pores.
According to the structure of the conventional hydrodynamic bearing device shown in FIG. 9, it is necessary to insert the sleeve 101 inside the sleeve cover 114 in the manufacturing process, which increases the number of man-hours in production. Since the sleeve 101 and the sleeve cover 111 are individual parts, the number of parts increases and the cost is also increased. In addition, if the sleeve 101 is inserted in the sleeve cover 114 with an inclined position, as shown in FIG. 10 in the insertion process, the axis of the bearing bore 101a is not kept perpendicular to the surface of the thrust plate 104. In this state, the gap of the thrust bearing or the radial bearing shown in FIG. 9 becomes uneven so that the shaft 102 cannot be supported in a stable manner. If the unevenness of the gap increases, the shaft 102 contacts the bearing bore 101a of the sleeve 101 and the bearing is seized up. The same problem can occur if the axis of the bearing bore 101a of the sleeve 101 is not precisely perpendicular to the opposed face of the thrust flange 103 fixed to the shaft 102.
When the shaft 102 rotates in the conventional hydrodynamic bearing described above, a hydraulic pressure within the range of 2-5 atmospheric pressures is generated by the radial dynamic pressure generating grooves 109a and 109b. If the oil is driven by this hydraulic pressure to flow in the pores of the sleeve 101, the hydraulic pressure is reduced to 70% of the above-mentioned pressure. As a result, stiffness of the radial bearing is also reduced to approximately 70%. Japanese Unexamined Patent Publication No. 2003-322145 discloses a method of covering the entire surface of the sleeve 101 with a coating layer that is not permeable to oil in order to prevent the oil from entering the pores of the sleeve 101. This method includes a step of forming the coating layer. Consequently, the method has many steps and a high cost.